Sen. Claire McCaskill, D-Mo., will not run for governor in the Show-Me State in 2016 — an open-seat race to replace term-limited Gov. Jay Nixon.

"It is a firm no," McCaskill told KCUR radio Monday about her gubernatorial ambitions. McCaskill said she will support another Democrat, state Attorney General Chris Koster, who announced his bid months ago, according to the Kansas City Star . She said she enjoys her job in the Senate, where she said she can have a big impact as a moderate member.

"I now have significant seniority. I now am part of a much smaller group holding down the middle," McCaskill said. "So I look at the makeup of the current U.S. Senate and I see what might be possible in terms of me helping forge some of those compromises, and at the end of the day the work is too important, the job is too rewarding and too fulfilling, ... so I will not be running for governor in 2016."

McCaskill is up for re-election to the Senate in 2018. She said it's "very likely" she'll run for a third term, citing her seniority.

But a 2018 re-election bid in Missouri could be an uphill battle for the two-term Democrat. Republican presidential candidates carried the state in 2008 and 2012. What's more, the 2018 cycle will be another midterm cycle.

In the most recent midterms in 2014, the GOP took control of the Senate for the first time since 2006 as several Democratic senators lost re-election in conservative states.

McCaskill won re-election by a stunning 16-point margin in 2012, in large part thanks to a weak GOP opponent, former Rep. Todd Akin. Akin's blunder-riddled campaign and controversial comments on rape have become a test case for how not to run a Senate race.

"I like being the underdog," McCaskill said, adding that she will begin "seriously raising money" for that bid this year.

"There's only about eight or nine moderates left in the Democratic caucus," McCaskill added. "And those of us that are still here that weren't defeated this last year are very important, because Republicans can't get anything done if they don't have any of us."